In any wireless communication system, various distortions are generated during signal transmission and reception. Such distortions may be caused by various components in the reception and transmission paths or by the radio-air interface. These distortions can significantly degrade communication system performance if not properly compensated.
FIG. 1 illustrates a typical wireless communication system. A signal to be transmitted is encoded in a Source Encoder 100 and a Channel Encoder 200, then modulated in a Digital Modulator 300. The encoded signal can be modulated according to any known modulation technique. For example, in the Personal Wireless Telecommunications Interoperability Standard (PWT), as described in Part 2: Physical Layer, TIA/EIA 662-2, a signal is .pi./4 Differential Quadrature Phase Shift Keyed (DQPSK) modulated. The Digital Modulator 300 is typically implemented in a digital circuitry. The modulated signal is then passed through a Transmission Channel 400 before being transmitted through the air via an Antenna 500. A series of bandpass filters are typically employed in the Transmission Channel 400 to assure that the signal to be transmitted is confined within a pre-defined frequency band with appropriate transmit characteristics.
The transmitted signal is received at an Antenna 600, processed through a Receiving Channel 700, which has similar circuitry as the Transmission Channel 400, demodulated in a Digital Demodulator 800, and decoded in a Channel Decoder 900 and a Source Decoder 1000. Ideally, the output from the Source Decoder 1000 is the same as the input to the Source Encoder 100.
FIG. 2 illustrates a detailed block diagram of the Transmission Channel 400. As shown in FIG. 2, the Transmission Channel 400 includes a D/A Intermediate Frequency (IF) Bandpass Filters 420 and 440, Mixers 430 and 450, and a Radio Frequency (RF) Front End 460. The IF Filters 420 and 440 confine the signal to a particular frequency band, the Mixers 430 and 450 up convert the baseband modulated signal to an intermediate frequency, and the RF Front End 460 converts the up converted signal to a radio frequency. The IF Filter 420 is typically an interstage filter that is centered, for example, at 11.25 MHZ, and the IF Filter 440 is typically a SAW filter that is centered, for example, at 422.5 MHZ.
The IF Filters 420 and 440 are typically designed with analog components or surface acoustic wave technology. Due to their analog nature, the IF Filters 420 and 440 often produce imperfect frequency responses which cause channel distortion. The channel distortion degrades the quality of the transmitted signal.
Reducing channel distortion to a reasonable level has always been a great challenge in wireless communication system design. Traditionally, the problem of radio channel distortion has been solved by simply putting more restrictive requirements on the analog filter design in the transmission channel. However, the design of a perfect analog filter that meets radio transmission requirements can be technically difficult. This often results in more expensive components and a longer design cycle. It is often not feasible to obtain an optimal analog filter design due to cost and time constraints.
Digital compensation provides an attractive alternative. For example, the Digital Cordless Telephone (DCT) 1900 modem includes a digital compensation filter. However, this filter is primarily concerned with compensating distortion due to signal digitization.
The problem of channel distortion is aggravated by the fact that components of the transmission system, such as the IF Filters 420 and 440, are typically temperature sensitive. That is, the frequency responses of these filters vary when the temperature in which the filters operate fluctuates. In wireless communication applications, systems are expected to perform in a wide range of temperatures. Base stations, for example, often operate in an outdoor environment or other environment in which the temperature is not controlled for personal comfort. In such an environment, the temperature variation over time can be quite substantial. For example, according to the Personal Wireless Telecommunications (PWT) Interoperability Standard, the class E2 temperature requirements for fixed parts (FP), radio fixed parts (RFP), and central control fixed parts (CCFP) is between -10.degree. C. and 55.degree. C. Other standards, such as IS136 and IS95, have similar requirements for system operation temperature. To assure optimal system performance, radio distortion should, preferably, be compensated over a wide range of temperatures.
Correcting temperature dependent radio distortion by analog means is simply too expensive to be a practical solution in commercial wireless communication applications. Conventional filtering techniques do not provide a practical means by which distortion can be compensated for over a wide range of temperatures.
It would be desirable to provide a digital compensation filter for a wireless communication system which compensates for radio distortion over a wide range of temperatures without requiring additional hardware.